The molecular structure of 11-methylbenz[a]anthracene: an almost planar substituted benz[a]anthracene.

The molecular structure of the almost inactive 11-methylbenz[a]anthracene has been determined by single-crystal X-ray diffraction analysis and refined to a final discrepancy index R of 0.036 over 955 independent reflections. With the benzo-ring A inclined at only 1.8 degrees to the furthest anthracene ring D, the benz[a]anthracene nucleus is much more nearly planar than in other (and carcinogenically more active) benz[a]anthracenes with bay-methyl substituents. Several carbon atoms in rings D and A deviate by approximately 0.05 A from the mean plane through the ring-carbon atoms of the bay-ring B. The shortest carbon-carbon bonds are C5-C6 = 1.341(5) A (at the K-region), C3-C4 = 1.355(5) and C8-C9 = 1.360(5) A, while C10-C11 next to the substituent is also quite short at 1.367(5) A.